Re: [PATCH v7 2/2] schemas: Add some common reserved-memory usages

[Simon Glass]

Hi Ard,

On Fri, 6 Oct 2023 at 17:00, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Fri, 6 Oct 2023 at 20:17, Simon Glass <sjg@xxxxxxxxxxxx> wrote:
> >
> > Hi Ard,
> >
> > On Fri, 6 Oct 2023 at 11:33, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> > >
> > > On Mon, 2 Oct 2023 at 19:54, Simon Glass <sjg@xxxxxxxxxxxx> wrote:
> > > >
> > > > Hi Rob,
> > > >
> > > > On Tue, 26 Sept 2023 at 13:42, Simon Glass <sjg@xxxxxxxxxxxx> wrote:
> > > > >
> > > > > It is common to split firmware into 'Platform Init', which does the
> > > > > initial hardware setup and a "Payload" which selects the OS to be booted.
> > > > > Thus an handover interface is required between these two pieces.
> > > > >
> > > > > Where UEFI boot-time services are not available, but UEFI firmware is
> > > > > present on either side of this interface, information about memory usage
> > > > > and attributes must be presented to the "Payload" in some form.
> > > > >
> > > > > This aims to provide an small schema addition for the memory mapping
> > > > > needed to keep these two pieces working together well.
> > > > >
> > > > > Signed-off-by: Simon Glass <sjg@xxxxxxxxxxxx>
> > > > > ---
> > > > >
> > > > > Changes in v7:
> > > > > - Rename acpi-reclaim to acpi
> > > > > - Drop individual mention of when memory can be reclaimed
> > > > > - Rewrite the item descriptions
> > > > > - Add back the UEFI text (with trepidation)
> > > >
> > > > I am again checking on this series. Can it be applied, please?
> > > >
> > >
> > > Apologies for the delay in response. I have been away.
> >
> > OK, I hope you had a nice trip.
> >
>
> Thanks, it was wonderful!
>
> > >
> > > >
> > > > >
> > > > > Changes in v6:
> > > > > - Drop mention of UEFI
> > > > > - Use compatible strings instead of node names
> > > > >
> > > > > Changes in v5:
> > > > > - Drop the memory-map node (should have done that in v4)
> > > > > - Tidy up schema a bit
> > > > >
> > > > > Changes in v4:
> > > > > - Make use of the reserved-memory node instead of creating a new one
> > > > >
> > > > > Changes in v3:
> > > > > - Reword commit message again
> > > > > - cc a lot more people, from the FFI patch
> > > > > - Split out the attributes into the /memory nodes
> > > > >
> > > > > Changes in v2:
> > > > > - Reword commit message
> > > > >
> > > > >  .../reserved-memory/common-reserved.yaml      | 71 +++++++++++++++++++
> > > > >  1 file changed, 71 insertions(+)
> > > > >  create mode 100644 dtschema/schemas/reserved-memory/common-reserved.yaml
> > > > >
> > > > > diff --git a/dtschema/schemas/reserved-memory/common-reserved.yaml b/dtschema/schemas/reserved-memory/common-reserved.yaml
> > > > > new file mode 100644
> > > > > index 0000000..f7fbdfd
> > > > > --- /dev/null
> > > > > +++ b/dtschema/schemas/reserved-memory/common-reserved.yaml
> > > > > @@ -0,0 +1,71 @@
> > > > > +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
> > > > > +%YAML 1.2
> > > > > +---
> > > > > +$id: http://devicetree.org/schemas/reserved-memory/common-reserved.yaml#
> > > > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > > > +
> > > > > +title: Common memory reservations
> > > > > +
> > > > > +description: |
> > > > > +  Specifies that the reserved memory region can be used for the purpose
> > > > > +  indicated by its compatible string.
> > > > > +
> > > > > +  Clients may reuse this reserved memory if they understand what it is for,
> > > > > +  subject to the notes below.
> > > > > +
> > > > > +maintainers:
> > > > > +  - Simon Glass <sjg@xxxxxxxxxxxx>
> > > > > +
> > > > > +allOf:
> > > > > +  - $ref: reserved-memory.yaml
> > > > > +
> > > > > +properties:
> > > > > +  compatible:
> > > > > +    description: |
> > > > > +      This describes some common memory reservations, with the compatible
> > > > > +      string indicating what it is used for:
> > > > > +
> > > > > +         acpi: Advanced Configuration and Power Interface (ACPI) tables
> > > > > +         acpi-nvs: ACPI Non-Volatile-Sleeping Memory (NVS). This is reserved by
> > > > > +           the firmware for its use and is required to be saved and restored
> > > > > +           across an NVS sleep
> > > > > +         boot-code: Contains code used for booting which is not needed by the OS
> > > > > +         boot-code: Contains data used for booting which is not needed by the OS
> > > > > +         runtime-code: Contains code used for interacting with the system when
> > > > > +           running the OS
> > > > > +         runtime-data: Contains data used for interacting with the system when
> > > > > +           running the OS
> > > > > +
> > > > > +    enum:
> > > > > +      - acpi
> > > > > +      - acpi-nvs
> > > > > +      - boot-code
> > > > > +      - boot-data
> > > > > +      - runtime-code
> > > > > +      - runtime-data
> > > > > +
> > >
> > > As I mentioned a few times already, I don't think these compatibles
> > > should be introduced here.
> > >
> > > A reserved region has a specific purpose, and the compatible should be
> > > more descriptive than the enum above. If the consumer does not
> > > understand this purpose, it should simply treat the memory as reserved
> > > and not touch it. Alternatively, these regions can be referenced from
> > > other DT nodes using phandles if needed.
> >
> > We still need some description of what these regions are used for, so
> > that the payload can use the correct regions. I do not have any other
> > solution to this problem. We are in v7 at present. At least explain
> > where you want the compatible strings to be introduced.
> >
>
> My point is really that by themselves, these regions are not usable by
> either a payload or an OS that consumes this information. Unless there
> is some other information being provided (via DT I imagine) that
> describes how these things are supposed to be used, they are nothing
> more than memory reservations that should be honored, and providing
> this arbitrary set of labels is unnecessary.
>
> > What sort of extra detail are you looking for? Please be specific and
> > preferably add some suggestions so I can close this out ASAP.
> >
>
> A payload or OS can do nothing with a memory reservation called
> 'runtime-code' it it doesn't know what is inside. So there is another
> DT node somewhere that describes this, and that can simply point to
> this region (via a phandle) if it needs to describe the
> correspondence. This is more idiomatic for DT afaik (but I am not the
> expert).  But more importantly, it avoids overloading some vague
> labels with behavior (e.g., executable permissions for code regions)
> that should only be displayed for regions with a particular use,
> rather than for a ill defined class of reservations the purpose of
> which is not clear.
>
> What I am trying to avoid is the OS ending up being forced to consume
> this information in parallel to the EFI memory map, and having to
> reconcile them. I'd be much happier if this gets contributed to a spec
> that only covers firmware-to-firmware, and is prevented from leaking
> into the OS facing interface.

I don't know about "another DT node". We don't have one at present.

There is already a note in the DT spec about this:

> 3.5.4 /reserved-memory and UEFI

> When booting via [UEFI], static /reserved-memory regions must also be listed in the system memory map obtained
> via the GetMemoryMap() UEFI boot time service as defined in [UEFI] § 7.2. The reserved memory regions need to be
> included in the UEFI memory map to protect against allocations by UEFI applications.
>
> Reserved regions with the no-map property must be listed in the memory map with type EfiReservedMemoryType. All
> other reserved regions must be listed with type EfiBootServicesData.
>
> Dynamic reserved memory regions must not be listed in the [UEFI] memory map because they are allocated by the OS
> after exiting firmware boot services.

I don't fully understand what all that means, but does it cover your concern?

Regards,
Simon